The present invention relates to the field of fiber amplifiers and lasers and to devices for optically pumping these in order to create a population inversion within an active medium of such amplifiers and/or lasers.
Devices for optically pumping fiber amplifiers and lasers are currently being used in time division and wave length division multiplex fiber communication systems, free space communication systems, remote measurement and sensing, scientific and laboratory experimentation and other applications.
One problem in the design of fiber amplifiers and lasers is to administer pump light to the active medium of the fibers with a power intensity sufficient to produce a reasonable gain.
Transversal pumping schemes popular with lasers in which the active medium has cross section dimensions of at least several millimeters are not straightforwardly applicable to fiber amplifiers and lasers. Due to the small cross section, overall absorption of transverse pumping light in the fiber is low, resulting in poor efficiency. On the other hand, absorption cannot be increased by increasing the concentration of the active species within the host material of the active medium since this may lead to thermal problems, to a narrowing of the gain vs. wavelength distribution and to other undesirable effects.
Accordingly, setups for longitudinally pumping fiber amplifiers and lasers have been studied. For example, in L. Goldberg, I. P. Koplow, D. Kliner, xe2x80x9cHighly efficient 4-W Yb-doped fiber amplifier pumped by a broad stripe laser diodexe2x80x9d, Optics Letters, volume 24, pages 673 to 675, 1999, it has been shown that longitudinal pumping of a double cladding fiber structure is an effective approach for constructing high power fiber amplifiers and lasers. Similar pumping setups are known from U.S. Pat. No. 4,815,079, J. D. Minelli et al., xe2x80x9cEfficient cladding pumping of an Er fibrexe2x80x9d, paper Th.L. 1.2, Proceedings of 21st European Conference on Optical Communications, Brussels 1995, and P. Bousselet et al., 26 dBm output power from an engineered cladding-pumped Yb-free EDFA for L-band WDM applicationsxe2x80x9d, paper WG5-1, Optical Fiber Conference San Francisco 2000.
Since the typical dimension of the inner cladding of such a fiber is 100 to 150 xcexcm, non-diffraction limited emission from high power broad area laser diode pumps can be efficiently coupled into such fibers. A 100 xcexcm wide broad stripe laser diode can generate an output power of 2 to 4 W at 810, 915 or 980 nm with long operating life.
The pump absorption coefficient of a double cladding fiber is inversely proportional to the ratio between the inner cladding area and the core area, which is typically in the range of 200:1 to 500:1. Because of this large ratio it is necessary to use very high dopant densities in order to achieve an adequate pump absorption coefficient, which is required to construct a reasonably short amplifier. Excessively long fiber amplifiers are not desirable because of background propagation losses in the gain fiber, increased cost, and signal distortion and interaction caused by nonlinear effects in the fiber core.
High doping densities, can be used with certain dopants, such as Yb, where a concentration of 104 to 2xc3x97104 ppm results in a typical cladding absorption coefficient of 1-3 dB/m at the peak Yb absorption wavelength of 975 nm. Erbium is more desirable as an active species for fiber amplifiers due to its usable gain band of 1530 to 1600 nm within the maximum transmission window of common fiber-glass compositions. However, it has been shown by P. Myslinski, D. Nguyen and J. Chrostowski, xe2x80x9cEffects of Concentration on performance of Erbium-doped Fiber Amplifiersxe2x80x9d, J. Lightwave Tech., v. 15, pp. 112-119 (1997) that in case of Er the concentrations must be kept at least ten times smaller (typically below 1000 ppm) than these values to avoid concentration quenching effects which significantly reduce amplifier quantum efficiency.
A known technology for circumventing this deficiency is Er/Yb co-doping (cf. J. E. Townsend, W. L. Bames, K. P. Jedrzejewski, S. G. Grabb, xe2x80x9cYb-sensitized Er Doped Silica Optical Fiber with High Transfer Efficiency and Gainxe2x80x9d, Electronics Lett. v. 27, pp. 1958-1959, (1991) where high Yb concentration (10-20 times that of Er) is used to achieve high pump absorption, and energy absorbed by Yb is efficiently transferred to Er ions. To achieve this efficient transfer, however, the core glass composition needs to have a large P2O5 content. Such gain fibers are difficult to fabricate with low background loss and high quantum efficiency. In addition the presence of P2O5 in the host glass causes substantial narrowing of the gain vs. wavelength distribution, and results in a small signal gain distribution that is much less uniform than that of amplifiers with Er-doped silica host glass cores. This gain non-uniformity makes the Er/Yb co-doped amplifiers unsuitable for WDM applications that require relatively flat gain distribution across a wide wavelength span.
An attractive method for constructing high power Er-doped amplifiers is to use double cladding structures having a small-inner-cladding. Such devices are described in J. D. Minelli, Z. J. Chen, R. L. Laming, J. D. Caplen, xe2x80x9cEfficient cladding pumping of an Er fibrexe2x80x9d, paper Th.L. 1.2, Proceedings of 21st European Conference on Optical Communications, Brussels 1995, and in P. Bousselet, M. Bettiati, L. Gasca, M. Goix, F. Boubal, C. Sinet, F. Leplingard, D, Bayart, xe2x80x9c26 dBm output power from an engineered cladding-pumped Yb-freeEDFA for L-band WDM applicationsxe2x80x9d, paper WG5-1, Optical Fiber Conference, San Francisco, 2000.
A small cladding, and a correspondingly small cladding-to-core area ratio make it possible to achieve high pump absorption with reasonably low concentrations. A maximum Er concentration of approximately 1000 ppm, possible with a host glass with a large Al2O3 content, would result in a core absorption coefficient of approximately 20 dB/m. In a double cladding fiber with an area ratio of A(cladding):A(core)=25:1 to 100:1, this would result in a cladding absorption coefficient of 0.8-0.2 dB/m. Assuming 90% pump absorption, these absorption coefficients are sufficiently high to construct efficient amplifiers with a length of 12 to 60 m, comparable to that of conventional, core-pumped amplifiers. Assuming a typical core diameter of 5 xcexcm these cladding-to-core area ratios require an inner cladding diameter of 25-50 xcexcm. The upper limit of core diameter, as dictated by bend losses and mode mismatch losses when fusion splicing to conventional transmission fibers, is represented by a 0.1 NA core of 10 xcexcm, corresponding to a cutoff wavelength of approximately 1.45 xcexcm. Such a core size would allow larger cladding diameters and/or larger absorption coefficients.
In addition to achieving sufficiently high pump absorption in double cladding fibers, another issue that must be addressed is whether a sufficiently high pump intensity can be achieved to produce a population inversion level required for high gain across the entire usable gain band of the active species. For Er, the threshold pump intensity, required for a 50% population inversion (50% of the ion population is in the upper state 4I13/2) is given by Ith=hv/st=10 kW/cm2, where hv is the photon energy, s=2xc3x9710xe2x88x9221 cm2 @ 980 nm is the absorption cross-section and txcx9c10 ms is the upper state lifetime. To achieve high gain and low noise performance in the amplifier, a population inversion of  greater than 80% is desirable, requiring a pump intensity that is approximately 5 Ith, or 50 kW/cm2. For a cladding pumped fiber amplifier with a 50 xcexcm cladding diameter, this requires a local pump power of 1 W. Since in an efficient amplifier the fiber needs to be sufficiently long to allow almost complete pump light absorption, the local pump intensity decreases exponentially as a function of position, causing a decrease in the local population inversion with distance from the front end of the amplifier. As a result, the pump power has to be larger by a factor of 2-3 than the value calculated above for the case of uniform pump intensity distribution. It is difficult to couple such power levels into an end surface of a double cladding amplifier fiber with little loss and with a focus tight enough to ensure that the pump power is essentially coupled into the inner cladding layer and at the same time to avoid damage to the end surface of the fiber due to excessive incident power.
The present invention is a method and apparatus that provides a double cladding fiber taper and an optical pumping device through which particularly high pump power densities can be coupled into a cladding layer of an optical fiber. Embodiments of the present invention maintain a high-level of packaging or coupling alignment tolerance so that they may be easily installed and used in an environmentally insensitive fiber amplifier.
An embodiment of the invention provides an optical pumping device by which pump light at particularly high power densities can be coupled into a cladding of an amplifying fiber, and light amplified by the fiber or to be amplified by the fiber can be coupled into and out of the fiber without a significant modification of the mode structure.
An embodiment of the invention provides an optical pumping device in which the light from a light source is gathered with high efficiency and is coupled into the amplifier fiber with low loss.
An embodiment of the invention provides an optical fiber amplifier and an optical fiber laser that achieve a high gain over a small length of fiber.
An embodiment of invention concerns an optical pumping device for pumping a fiber amplifier or laser, the device comprising a light source and a light guiding section, the light guiding section comprising a first cladding layer surrounding a fiber core. The first cladding layer has a wide diameter portion, a narrow diameter portion and a tapered portion connecting the wide and narrow diameter portions. The light source is arranged to couple light into the first cladding layer at the wide diameter portion. At this portion, a relatively large surface is available for coupling the pump light into the cladding layer at moderate power densities. By having this pump light propagate along the tapered portion towards the narrow diameter portion, the power density of the pump light may be increased to a level that will result in a high dopant population inversion.
A typical diameter ratio of the wide diameter portion to the narrow diameter portion may be in a range of 2:1 to 6:1. Under proper conditions, such a taper concentrates the pump light injected into the wide end of the taper into the narrow end of the taper, thereby achieving an increase in the pump intensity equal to the square of the taper ratio, or a factor of 4 to 36.
In order to avoid any modification in the mode structure of light propagating in a core of the amplifying fiber, it is preferable that the diameter of the fiber core of the light guiding section of the optical pumping device is the same in the wide diameter, tapered and narrow diameter portions.
The ratio of diameters of the first cladding layer and the core in the narrow diameter portion is preferably in a range from 1.5:1 to 10:1, small ratios being preferred for shorter amplifiers. Other ratios, outside of this range may be used.
According to a preferred embodiment, a lateral v-shaped groove is formed in the first cladding layer at the wide diameter portion, and the light source is arranged to irradiate the groove. In particular, pump light from the light source may be incident on a facet of the groove from a side of the wide diameter portion opposite that of the groove. In this way, light incident at a near normal angle with respect to an axis of the light guiding portion may be reflected towards the tapered portion along said axis or at small angles relative to the axis.
In order to avoid loss of pump light during its propagation through the tapered portion, at the pump light may be coupled into the wide diameter portion with a small numerical aperture of 0.05 to 0.2.
For this purpose, a first lens may be provided for gathering light from the light source with a large first numerical aperture in a first plane and focusing it onto the facet of the groove with a second numerical aperture smaller than the first one, preferably in the range of 0.05 to 0.2.
As a pump light source, a broad stripe laser diode may be used. In contrast to low power (100-200 mW) diffraction limited laser diodes having stripe widths of 2-5 micrometers, this type of high power laser diode, typically has a stripe width of 100-200 micrometers which allows it to generate 2-4 W of power.
Since such a laser diode has different angular spreads of its output beam in its junction plane and in a plane perpendicular thereto, a second lens may be provided for gathering light from the light source with a third numerical aperture in a second plane and focusing it onto the facet of the groove with a fourth numerical aperture, the third numerical aperture being smaller than the first one and the second and fourth numerical apertures being approximately the same.
In an embodiment, the first and second lenses are crossed cylindrical lenses.
According to an alternative embodiment, the light source is arranged to couple light into an end surface of the wide diameter portion of the light guiding section. For this purpose, a dichroic mirror may be provided having a reflectivity adapted to combine and/or separate pump light from the light source and a light at an active wavelength of the fiber amplifier or laser.
An optical fiber amplifier may be formed by connecting, preferably fusion splicing, a first end of a gain fiber comprising a first cladding layer and a doped core to the narrow diameter portion of the light guiding section of a pumping device as defined above, in order to propagate pump light from the pump light source through the first cladding layer of the light guiding section into the first cladding layer of the gain fiber.
Preferably, in this optical fiber amplifier, the gain fiber has a second cladding layer surrounding the first cladding layer, the second cladding layer having a lower refractive index than the that of the first cladding layer. In this way pump light losses are avoided which would inevitably occur if the first cladding were directly surrounded by a fiber jacket that typically has a higher refractive index than the cladding. Of course, the number of cladding layers of the gain fiber is not limited to two.
In order to achieve a homogeneous pump power distribution over the length of the gain fiber, a second pumping device of the type defined above can be connected to a second end of the gain fiber, or further pumping devices may be inserted at intermediate locations of the gain fiber.
A dopant of the doped core of the gain fiber is preferably selected from a group of rare earth ions such as Er, Yb, Er/Yb, Nd, and Tm. For amplifiers having a gain distribution in the 1.5 xcexcm range, the doped core may be doped with Er only, so that the core glass may have a small component of P2O5.
A fiber laser having features and advantages similar to those of the optical fiber amplifier explained above can be obtained if the optical gain fiber is equipped with reflectors at its ends.
Further features and, advantages of the present invention will become apparent from the subsequent description, by word and drawing, of preferred embodiments of the present invention.